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Short Version:
Your sediment filter should not last “forever”, a dirty sediment filter is a “good thing” (its working!), change your sediment filter when it drops approximately 20% of the inlet pressure, or sooner if your production rate is not sufficient, or if you have a low pressure supply (less than 50 pounds).

Long version:
In general, a Sediment Filter will usually last approx. 4-6 months, depending on the micron size rating (usually the smaller the better, for example common sizes include ten, five, one, “half” 0.5 micron, and the best are as small as 0.2 microns), the volume of water passed through your filter, and the quality of your water. Water quality factors include turbidity, iron content, organics, and total particulate volume in your water source.

If you have considerable particulates in your water, your filter may plug up quicker if it is sized properly and is doing its job (stopping particulates from passing thru) as compared to if you have an incorrectly sized filter; the filter is not properly seated in the housing, or is of poor or inadequate construction. We have all heard stories of somebody saying their sediment filter lasted for “years”, but rarely are we provided useful information like how much water were they producing, how much sediment was in their water, how long was their carbon filter or membrane lasting, or what quality of water were they producing for us to get very excited. Sure, there are a few places left in the USA that have phenomenal water quality and in these few places sediment filters last much longer, but that is the pretty rare (and unfortunately seems to be getting worse). In general, you get what you pay for and making sure you have a quality sediment filter is critical to the life and performance of almost every component downstream in your system. Don’t buy a Cadillac then use a cheap oil filter is one of the old anecdotes that applies.

If you have a water source with particulates smaller than the rated size of the filter, it may not stop these particulates, rather it will instead pass them through to the next stage of filtration (usually a carbon filter) which in turn will become blocked prematurely (and/or the particulates will occupy valuable active carbon surface area that should be used to remove volatile organics, solvents and other chemicals that carbon filters are designed to remove. We call this “using a carbon filter as a sediment filter” or… “a bad idea”). Keep in mind, sediment filters are supposed to stop sediments from passing through. If they do a poor job of it, they may “last” a long time, but then your next stage of filtration has even more work to do. Also, since carbon filter condition is critical to protecting your RO membrane life, a poor or improperly selected sediment filter can easily compromise the life/performance of your membrane. Membrane performance (rejection) is crucial to longevity of your DI stages.

Note, while your municipality may provide local water quality information, in many places the water quality at the point that you have connected your system can be quite different, even from that of a neighbor! Additionally, water quality varies in most locations seasonally, or from other events (new construction, municipal operation changes, droughts, floods, etc.). So in general it is usually pretty hard to compare your sediment problems or filter life spans with other people, even within the same city! While a rule of thumb is “change your pre-filter(s) every six months” your situation may require quite a different schedule. Thus our recommendation is to use a Pressure Gauge to determine when your Sediment Pre-Filter needs replacement. This is a relatively easy way to determine if it is time to change your filter! Basically, when you measure a drop in pressure between 15-20% from where your normal house (inlet) pressure is, it is time to replace the filter. Note that a drop in the system’s production (output) is “in most cases” an indication that the sediment filter has become saturated with contaminants and will need to be replaced. If you remove the sediment and the pressure does not return to normal, the carbon filter may also be plugged. Also note that if your water contains a great deal of sediment or chlorine, the pre-filters may require more frequent changes to maintain adequate production rate and extended membrane life. In general, for adequate production and sufficient membrane rejection pressures above 50 pounds are desirable. So, if your inlet pressure is already below 50 pounds, you may want to change your sediment filter at 10% in order to maintain at least 45 pounds pressure on your membrane (50 minus 10% (of 50), or about 45 pounds).

To check this, briefly run water through the system without the filter in its housing. If the pressure jumps back to your normal house pressure without the filter, you know the filter you just took out was plugged up.

Carbon filter changes:

Short version:
your carbon filter should not last “forever”, a plugged carbon filter dropping significant pressure is a “bad thing” (it’s not removing chemicals when loaded with particulates!), change your carbon filter when the chlorine level is above 0.1ppm using a simple chlorine test kit.

Long version:
In general, a carbon Filter will usually last approx. 4-6 months, depending on the micron size rating (usually the smaller the better, for example common sizes include ten, five, one, with some as small as “half” 0.5 micron), the “capacity” rating (usually expressed in ppm Gallons, but more on this later), the volume of water passed through your filter, and the quality of your water (see sediment filter changes above). Water quality factors include chlorine level, although depending on your sediment filter status, they may also include turbidity, iron content, organics, and total particulate volume in your water source.
While a rule of thumb is “change your carbon filter every six months”, your situation like that described above in “changing your sediment filter” may require quite a different schedule. Also, note that we rarely recommend changing your carbon filter by the “pressure drop” method like that recommended for the sediment filter. Why? Because carbon filters are designed to optimally remove chemicals from water, not remove sediments (although it will). Unfortunately, when a carbon filter is used to remove sediments it rarely reaches its useful chemical removal life (the reason it is in your system). With a good sediment filter a carbon filter has a much better chance of reaching its useful chemical removal life far before it significantly drops pressure. This is why we recommend that the best way to determine when your Carbon Block Pre-Filter needs replacement is to use a simple chlorine test kit. Why test for Chlorine? It has been well established by RO Membrane manufacturers that for most systems a chlorine level above 0.1 part per million (ppm) will cause damage to the RO (reverse osmosis) membrane. Thus when you determine that your chlorine level is above 0.1 ppm it is time to change your carbon filter. Also, most aquarists are trying to remove Chorine from their feed water, so the carbon filter is an important part of the system for achieving this goal.
I mentioned previously that many carbon filters not only have size ratings (1 micron, ½ micron etc) but also many have an advertised “capacity”, usually something along the lines of “6,000 gallons!” , “12,000 gallons” , or the like. These ratings are more specifically a rating of ppm gallons. So for example, the 6000 gallon rating is 6000 gallon when presented with a 1ppm chlorine challenge. What about pressure drop? Well, if it is still removing chlorine, acceptable pressure drop is determined by how much inlet pressure your system has and how much pressure is being presented to your membrane (usually at least 45 pounds is required, although 60+ is desirable for performance reasons). So, if you have 50 pounds inlet pressure, your sediment filter is dropping 5 pounds, and your carbon is dropping 5 pounds, you only have 40 pounds at the membrane, which is a little low. You can either add a booster pump (another discussion) or change your filters more frequently (or accept the lower performance of the membrane). So, now that you better understand the rating system, one can see how tap water levels of chorine can greatly affect the capacity or life of the carbon filter.

To test for chlorine breakthrough, collect a 10 ml sample of the concentrate from the yellow tubing and test the chlorine concentration using our test kit TK-CL-25. If the chlorine concentration is above 0.1ppm, replace the carbon pre-filter. A drop in the system’s production is “in most cases” an indication that the sediment filter has become saturated with contaminants, but a carbon filter can also drop production, if it’s covered with extremely fine sediment. If the carbon becomes plugged with sediment, it will no longer be able to remove chlorine.